1. Field of the Invention
The present invention relates to a gas generator used for an air bag safety system for assuring safety of a driver and passengers of a car from shock caused by car crash. Specifically to a gas generator for air bags (hereinafter referred to simply as a "gas generator") whose number of parts may be reduced and whose production cost may be lowered without impairing its performance as a gas generator.
2. Discussion of Background
As a gas generator of this sort, there has been one disclosed in Japanese Patent Application No. Hei. 7-333902 applied by the present applicant for example. As shown in FIG. 7, this gas generator has an upper container 101 made from aluminum having a double piped structure whose one end (upper end in the figure) is closed and a lower container 104 made from aluminum having a double short piped structure having an opening at the axial center part thereof. Then, they are friction welded by respectively butting edges of inner cylindrical walls 102 and 105 and edges of outer cylindrical walls 103 and 106 of the upper container 101 and the lower container 104. Accordingly, this gas generator has a housing structure in which a center spatial section and a ringed spatial section around that are created.
An ignition chamber P is created at the center spatial section by charging a squib 118 and enhancer 119 from the bottom. A retainer ring 116 is disposed and fixed within the ringed spatial section. An inner peripheral edge portion 116d and an outer peripheral edge portion 116e of the retainer ring 116 abut respectively with friction welding burrs 102b and 103b of the upper container 101. Then, gas generating agents 107 and a cooling member/filtering member (hereinafter referred to as a "cooling filter member") 110 are stored within the ringed spatial section interposed between the upper container 101 and the retainer ring 116. A combustion chamber G and a filtering chamber F are created by storing the gas generating agents 107 and the cooling filter member 110 radially in order from the inside.
Such a gas generator has an advantage that because its housing is jointed by friction welding the inner pipes and the outer pipes of the double pipes respectively from each other, reliability of joint strength is very high and the safety of the gas generator may be enhanced. Further, the friction welding has an advantage that it requires no expensive welding facility and its production cost can be lowered.
However, the friction welding method has had a problem that it is difficult to control height of welding in general. A special care is required especially when the gas generating agent which is one of propellants is charged inside such as the gas generator. When the height cannot be fully controlled for example, a phenomenon of that a lower cover 121 of the lower container 104 contacts with the cooling filter member 110 and the cooling filter member 110 rotates together with the lower container 104 occurs while rotating the lower container 104 in friction welding. That is, a phenomenon of so-called co-rotation occurs. When such a situation occurs, the gas generating agent 107 is rubbed by the inner face of the cooling filter member 110. As a result, there is a possibility that the gas generating agent is fired and explodes in the worst case depending on the type of the gas generating agent and the environment such as use temperature of the gas generator and it is very dangerous.
It is then necessary to facilitate the control of the height during the friction welding by assuring a clearance 140 between the cooling filter member 110 and the lower cover 121 of the lower container 104 in order to avoid such a situation from occurring. Then, because such clearance 140 is provided, it is necessary to maintain air-tightness of the combustion chamber G so that slag within combustion gas would not flow out of the end of the cooling filter member 110 when such gas generator is operative. From such necessity, the retainer ring 116 is disposed as an essential member so that the performance of the gas generator is not impaired.
However, in the present situation where air bag systems are being mounted as standard in vehicles, it has become an urgent requirement to lower the cost of the gas generator. Then, attaching much importance to that it is effective to reduce a number of parts to reduce the production cost, the inventors et. al. have studied ardently from such a point of view. As a result, the inventors have consummated the present invention.